478 Weed and Pirsson—Igneous Rocks of Montana. 
Quartz Minette Shonkinite 
syenite porphyry. Syenite Yogoite Sheep Shonkinite Square 
Big Baldy Mt. Yogo. Yogo. Creek. Yogo. Butte. 
SiO, 67°04 61°65 54°42 52°26 48°49 46°73 
ALO. 15:25 15°07 14°28 13°96 12°29 10°05 
Fe,O, 1°69 2°03 3°32 2°76 2°88 5 3°53 
FeO 1:13 2°25 4°13 | 4°45 5°77 8°20 
MgO 1°75 3°67 6°12 8°21 SIE). 9°68 
CaO Pg bp 4°61 CTR aw Uo 9°65 13°22 
Na,O 4°09 4°35 3°44 2°80 2°22 ish 
K,O 5°10 4°50 4°22 3°87 4°96 3°76 
The very regular gradation which this series shows is quite 
remarkable, as may be seen in the above table. There are very 
few irregularities in it. The variations are all along the same 
lines as those shown at Yogo Peak itself—the gradual fall of 
silica, alumina and soda together, with the predominance of 
potash over soda, may be taken as characteristic of this ‘* petro- 
graphical province.” From consideration of the analyses and 
the character of the differentiation that has taken place at 
Yogo Peak, together with the fact that the coarse mica type of 
shonkinite shows this differentiation in a higher degree than 
the type analyzed, it seems probable that it must agree in 
chemical composition with the Square Butte rock even more 
closely than the type analyzed does. 
Classification of the Yogo rocks.—Here we enter a vexed 
field. ‘Those who believe in classifying rocks solely by their 
structure and the kinds of minerals they contain without 
placing any importance upon the relative quantities present, or 
in other words place no stress upon the chemical composition 
of the magmas from which the rocks are derived, would doubt- 
less call all of the rock varieties at Yogo Peak syenites, since 
they are composed chiefly of augite and orthoclase. 
It seems to us that the time has come when a sharp distine- 
tion must be drawn between the use of general terms used by 
- field geologists, such as granite, porphyry, trap, greenstone, etc., 
and the more exact and definite nomenclature demanded by 
the needs of petrology. Such general terms have a definite 
and proper value just as tree, bush and vine have in botany, 
but the science of petrology demands at present a terminology 
which will not only be gualetative but also quantitative in its 
meanings. While it is neither possible nor desirable to classify 
rocks on a strictly chemical basis, it is clearly evident that the 
lines of mineral and consequently of chemical variation must 
be more strictly drawn than has hitherto been done. This 
will find its natural manifestation in a more strict regard to 
the relative quantities of the various minerals which are present 
and these quantities within reasonable limits must be expressed.* 
* See also the discussion of this subject by Brégger (Gest. der Grorudit-Tin- 
guait Serie, p. 91, with whose excellent presentation of the subject we, in the 
main, heartily agree). 
